1. Field of the Invention
This invention relates to charging circuitry for a combination trolley wire-battery powered locomotive and more particularly to a charging circuit having a voltage sensing device for monitoring the voltage level of the battery being charged to insure that the battery is charged to a maximum safe level.
2. Description of the Prior Art
Conventional mine locomotives receive power from a trolley wire that is suspended from the mine roof and extends through the haulageway of an underground mine. The energized trolley wire supplies direct current power to the electric motor and propels the mine locomotive on tracks through the mine. The trolley wire is suspended from the mine roof and is continually energized presenting a substantial hazard when equipment comes in accidental contact with the energized wire. Furthermore, trolley wire operation of mining locomotives becomes more troublesome the lower the coal seam becomes because of the limited overhead clearance available for suspending the trolley wire from the mine roof.
One solution to the problems presented by trolley wire operated locomotives is the combined use of trolley wire and battery power for propelling the locomotive. A combination or trolley-battery locomotive utilizes existing trolley wires on the main haulageway. In those locations in the mine where trolley wires do not exist, trolley operation of the locomotive is selectively switched to battery operation. This is particularly adaptable in the side entrys where it is not feasible for economic or safety reasons to install an overhead trolley system.
Because the life of a battery is limited, it must be recharged at periodic intervals requiring either removing the locomotive from operation and substituting a freshly charged locomotive or parking the locomotive during the recharging stage. A combination locomotive on the other hand has the advantage of allowing recharging of the batteries during the interval in which it is on trolley wire operation. If the ratio between trolley and battery operation is sufficiently great, the locomotive need not be parked for the purpose of charging the battery.
One approach to the combination locomotive utilizes a battery having 70 to 90 cells which are recharged when on trolley operation through onboard resistors. Accordingly, the charge current must be kept sufficiently low to prevent excessive heating of the resistors. In addition, a significant decrease in performance occurs when the operation is transferred from trolley power to battery power because of the drop in operation voltage between the two methods of operation.
One means of overcoming the drop in operating voltage between trolley operation and battery operation incorporates placing batteries in series during the trolley operation. This arrangement eliminates the need for an additional resistor when the batteries are then placed in parallel to drive the motors. The motors operate in series when on trolley power and in parallel when on battery power. However, with this arrangement, when one motor fails the other generally fails, too.
There is need for an apparatus for the operation of combination trolley and battery powdered mine locomotives which provides for continuous operation of the locomotive during a shift by transferring operation from the trolley wire to the battery without experiencing a decrease in motor performance. While it has been suggested to provide reliable combination locomotives utilizing trolley and battery operation, the prior art systems experience interruption in the locomotive operation in the recharging of the batteries and distinction in motor performance when transferred from trolley to battery operation.